Transition of the Flow Reversal in Turbulent Thermal Convection

We present an experimental study on the flow reversals in quasi-2D turbulent Rayleigh-Benard convection. It is found the reversal rate decreases with Rayleigh number (Ra) and there is a transitional Ra, which separates the slow decease and fast decease regime of the reversal rate. Our velocity measurements show that for low Ra the interior of the main roll of the flow has already breaks into two small rolls although they are still enclosed by the envelope of the main roll, which is the reason for the frequent reversals for low Ra. In addition we decomposed the instantaneous flow field into Fourier modes and defined the stability of the single-roll mode, found that the stability of the single-roll mode experiences a slow increase followed by a fast increase. We believe this transition of the single-roll stability is responsible for the transition of the reversal rate. We also studied the Pr dependence of the transition and successfully revealed why reversal happens only in a restricted parameter ranges.